The present invention relates to a device for injecting preheated air into a shaft furnace, composed of several tubular elements equipped with an inner refractory lining and connected, on one side, to the wall of the furnace by means of an elbow, a tuyere and a nozzle and, on the other side, to a main circular pipeline arranged round the furnace and fed with preheated air by means of a system comprising a blower, several Cowper stove and a mixing chamber.
A device of this type, known more generally as a "blastpipe holder" is known from U.S. Pat. No. 3,766,868. In certain high temperature furnaces, blastpipe holders are arranged in a relatively high number round the base of the furnace, for example a blast furnace, for the purpose of injecting hot air into it in order to maintain the combustion and reduction process in the furnace. To this end, the air is injected at a temperature of the order of 1,200.degree. C. or above, and the heating to this temperature is carried out in air heaters currently called "Cowper stoves". A device of this type is shown in U.S. Pat. No. 3,766,868, the disclosure of which is incorporated herein by reference.
Because of this high temperature of the air passing through the blastpipe holders, conventional blastpipe holders have a thick inner refractory lining, on the one hand to prevent heat losses and on the other hand to protect the outer metal casing against thermal radiation and the high temperature of the air. Unfortunately, the great thickness of this refractory lining has an adverse influence on the production cost of the blastpipe holders and, furthermore, increases the weight and overall size of these. Despite some attempts, it has not been possible hitherto to provide a more advantageous alternative.